I'm going there tomorrow. Curious to see if it's any different
Okay so here's my experience of Kickass Grassmarket in Edinburgh:
The hostel itself is great: great location, good amenities. Especially the private pods are amazing if you are someone that likes some privacy/alone time.
About the open showers:
They were bone dry around 8 PM, when you'd think most people would be showering. They're also tucked away in a remote corner of the building at the end of a long hallway. So I think it's not necessarily just people avoiding them but also a thing of people not knowing it's there at all.
Tldr: Kickass Grassmarket in Edinburgh is a great hostel but don't go for the communal showers, you'll be disappointed.